The present invention relates to invert emulsions for the well-drilling industry, to a method for their preparation and to an emulsifier composition useful therein. More particularly, the present invention relates to improved emulsions of a brine in a liquid hydrocarbon, and further comprising an improved silicone emulsifier composition, which are useful in well-drilling operations, such as in gas- and/or oil-well drilling, as a drilling fluid, a completion fluid, a workover fluid, a casing pack fluid, a fracturing fluid, a packer fluid and a spacer fluid.
Invert emulsions, in the form of drilling muds, completion fluids and packer fluids are well known in the well-drilling art. Invert emulsions typically consist of a fluid phase comprising from 15 to 45 percent by volume of a CaCl.sub.2 brine and 55 to 85 percent by volume of diesel oil, and a solid phase comprising agents for pressure control, filtration control, viscosity control, gelling, etc. An invert emulsion drilling mud generally contains from 5 to 30 percent by volume of the brine.
While conventional invert emulsions are used in drilling deep wells, where high temperature and pressure are commonly encountered, they are not completely satisfactory for such a use. For example, invert emulsions are sometimes not sufficiently stable at the high temperatures encountered in deep wells. Furthermore, the use of large amounts of a weighting agent in an invert emulsion to achieve pressure control in deep wells is often undesirable because the permeability of the producing zone can be adversely affected. Heavier brines, such as brines containing CaBr.sub.2 and/or ZnBr.sub.2, have been used to increase the density of silicone-free invert emulsions, thereby achieving greater pressure control; however, the stability of these heavier emulsions is marginal or non-existent at high temperature.
Thermally stable invert emulsions comprising a brine, a liquid hydrocarbon and a polydiorganosiloxane have been disclosed by Romenesko et al., U.S. Pat. No. 236,968, filed on Feb. 23, 1981 and assigned to the assignee of this invention. While the invert emulsions of Romenesko et al. constitute a significant advance in the art, some of them do not possess a high degree of thermal stability unless they are thickened with conventional thickening agents. In particular, the solids-free invert emulsions of Romenesko et al. comprising a heavy brine of CaBr.sub.2 and ZnBr.sub.2 seem to degrade at high temperature, apparently because HBr, which is produced by the heavy brine at high temperature, degrades the polydiorganosiloxane emulsifier. Invert emulsions, and particularly solids-free emulsions, comprising a heavy brine and having greater stability in downhole conditions are therefore still needed.